HYDRODYNAMIC CONSEQUENCES OF THE AMMONOID CONCH: SHAPE CHANGE THROUGH ONTOGENY

Aquatic animals’ complex interactions with moving water are greatly influenced by body shape, size, and velocity. Ammonoid cephalopods produced an unparalleled fossil record of external conchs in a wide variety of shapes, in which accretionary growth reveals the whole sequence of ontogenetic shape change (allometry) within a given individual animal. Varied shell shape-and-size combinations across an individual’s lifespan could have greatly influenced their potential swimming capabilities. Any generalizations about ammonoid swimming performance – for individuals, species, or morphotypes – must acknowledge the possible influence of ontogenetic change.

We investigated the hydrodynamic consequences of conch allometry throughout the ontogeny of several ammonoid morphotypes. Virtual 3D models were created from measurements of real taxa, then loaded into Computational Fluid Dynamics simulations to compare their hydrodynamic attributes at particular scales. We use the open source software, Blender, to model four morphotypes (an oxycone, serpenticone, sphaerocone, and an “averaged” morphospace center). The former three morphotypes are represented by the species Achguigites tafilaltensis, Dactylioceras commune, and Goniatites crenistria, respectively, while the center is a theoretical, isometric average. All adult conches were scaled to diameters of 10 cm, then cut back to diameters of 7, 4, and 1 cm to isolate different ontogenetic stages. By normalizing adult diameters, we are able to explore allometry in conch compression/inflation, and coiling parameters while isolating variables. Finally, we simulate flow velocities as a function of conch diameter to capture a range of Reynolds numbers (i.e., flow regimes dependent upon size and velocity).

Different shapes incur different hydrodynamic advantages and consequences at different scales. Our results demonstrate that ammonoid cephalopods may have navigated these scale-dependencies with allometric growth through ontogeny. Furthermore, these morphological differences may reflect ontogenetic shifts in the swimming capabilities and life habits of these animals, depending on a particular size.